Well for starters Dirks had been hurt for several games.
Also, a manager has to manage personalities as well as lineup cards. So if he makes Delmon a full time DH because a bunch of people want Dirks in LF, then you have a pissed off guy in the clubhouse, and that likely leads to more problems. Delmon does not want to DH, remember how bad it got when Sheff bitched all the time about having to DH?
When did Sheff bitch about DHing?